1. Field of the Invention
The present invention relates to detect defects on a disk surface, and more particularly, to a method and apparatus for detecting linear defects and circular arc defects that exist on a disk surface and periodically occur.
2. Description of the Related Art
As a material of an information recording medium or a semiconductor, a disk such as glass, silicon wafer, and the like is used. When there are defects on the surfaces of these materials, characteristics of a product are deteriorated. As a result, detection of these defects is performed by an apparatus for detecting defects on a disk surface. The apparatus for detecting defects on the disk surface detects the defects generated on the disk surface. There are various kinds of defects, for example, fine dust (particle) attached to the surface, spot (stain), abrasion mark (scratch) due to foreign materials, fine concave portions (pit) or convex portions (bump), smoothly inclined concave portions (dimple), change (handling damage) of a disk edge caused due to collision, etc., at the time of delivering a disk, a grinding mark (glide) of a disk surface, and the like. As a method for effectively detecting these various defects, an apparatus for detecting defects on a disk surface according to the related art uses a method that detects defects by irradiating a laser beam on the disk surface and receiving each optical characteristic differently detected in terms of shape, size and the like of each defect, that is, reflected light or scattered light of the laser beam. FIG. 20 shows a schematic configuration of the apparatus for detecting defects on a disk surface that is disclosed in JP-A-2000-180376 and JP Patent No. 3732980 using the above-mentioned detecting method.
In addition to the above-mentioned defects, a defect called ‘wrinkle shape’ (hereinafter, referred to as wrinkle defect) is formed on the disk surface. The wrinkle defect is a defect that occurs during a process when the disk is shrunk by heat while the disk is manufactured. FIGS. 2A to 2E show features of the shape. FIG. 2A shows the whole disk and FIGS. 2B and 2C each show enlarged parts α and β where the wrinkle defect occurs. Further, FIG. 2D shows a cross sectional profile of FIG. 2B, and FIG. 2E shows a cross sectional profile of FIG. 2C. The wrinkle defect, which is a defect that periodically generates rugged portions in a linear shape or a circular arc shape, is a low aspect defect of which a height is very low as compared to the occurrence period of the defects. If the above defect exists on the disk surface, it makes a floating amount of a head unstable and affects the accuracy of magnetic reading and writing operations. As a result, this defect is considered as a serious defect. Therefore, a disk having the wrinkle on its surface is considered as a defective disk and may be thus treated like defective goods.
The apparatus (FIG. 20) for detecting defects on a disk surface known from the related art performs the detection for a low aspect (surface ruggedness) defect by allowing a light receiving element 202 to receive specula reflection light from a second light transmitting system 201. FIG. 3 illustrates the principle of detecting the low aspect defect. The second light transmitting system 201 has a configuration to project a parallel light having a predetermined width on a surface of the disk 301 that can detect the defects, and the light receiving element 202 has a configuration to receive the specula reflection light through a filter 203 (FIG. 3A). When a concave defect 3011 exists on the surface of the disk 301 (FIG. 3B), characteristics of the concave defect are similar to those of a concave lens, such that parallel light can be collected in the light receiving element 202. At this time, since a signal level detected by the light receiving element 202 is amplified, as shown in FIG. 3D as the amount of light received is amplified, it is possible to detect the concave defect by, for example, a threshold 2023. Further, when a convex detect 3012 exists on the surface of the disk 301 (FIG. 3C), the convex defect is operated like a convex lens, such that parallel light can be diffused and collected in the light receiving element 202. At this time, since the signal level detected by the light receiving element 202 is reduced and amplified, as shown in FIG. 3E as the amount of light received is reduced, it is possible to detect the convex defect by, for example, a threshold 2024.
However, since the increase and decrease of the signal are extremely small in the wrinkle defect portion, it is difficult to detect the wrinkle defect having extremely low aspect ratio among the low aspect defects. When detecting the wrinkle defects using an apparatus for detecting defects on a disk surface according to the related art, there are two problems as follows. The first problem is that the apparatus for detecting defects on a disk surface according to the related art detects the disk in a spiral shape as shown in FIG. 4 and performs a threshold process using a one-dimensional signal as a signal process. In this process, when the defects periodically exist in a radial direction (r direction) like the defect A of FIG. 4A, if the defect is detected in the spiral shape, a periodic change in a signal like a signal of FIG. 4B, which is intersected in a width direction of the defect, is obtained. However, if the signal strength is extremely small, when the threshold process is performed on the signal, the signal change can be hidden in the defect portion A by the change in the signal strength due to the waviness of the disk.
In order to avoid this, a band pass filter, which passes through only a frequency band in which the defects exist, is generally applied to the signal, making it possible to remove the waviness and detect the defects caused during the threshold process. However, when the defects periodically exist in a circumferential direction (θ direction) like the defect B of FIG. 4A, if the defects are detected in the spiral shape, there is a case where the change in the signal in the defect portion B like a signal of FIG. 4B that is intersected in a length direction of the defect is changed to be approximately the same as the period of the waviness. In this case, it is difficult to detect the defects even by using the filter process.
The second problem is that even if there is the signal strength signal in the wrinkle defect portion like the defect portion A, since the change in the signal strength is extremely small, the defects may be overlooked due to the setting of the threshold or a large amount of false reports may be generated. Further, distinguishing between the kinds of defects having linear or circular arc features, even if they are detected, are insufficient.
There is a problem of having a bad effect on a hard disk or a problem of generating a large amount of false reports by overlooking the wrinkle defect such that it is very inconvenient as the apparatus for detecting defects on a disk surface.